Create new proof: Topology world lvl 1

[lnay]

Create a new lean file with the name specified below in Game/Levels/content_drafts which correctly compiles and defines and proves the theorem below. Break into intermediate theorems/lemmas if necessary (to go into separate levels, or provided as 'given' helpers, tbd later).
file name	topology_wrld_L03.lean
proof	unions of open sets are open in a metric space
extra info

Syllabus outline: #5